In recent years, treatment has been performed in which a semiconductor wafer or other substrate for treatment is affixed to a hard supporting substrate, after which the surface of the substrate for treatment opposite the affixed surface is treated. For example, Patent Reference 1 discloses a technique in which a substrate for treatment is affixed to glass, which is a supporting substrate, with a photothermal conversion layer and an adhesive interposed, grinding treatment is performed on the substrate for treatment to an extremely thin shape, and after grinding, the substrate for treatment is separated from the supporting substrate without being damaged.
This technique is explained referring to FIG. 16 through FIG. 22. As shown in FIG. 16, a mixed solution comprising carbon black or another optical absorbent and a thermally decomposable resin is applied using a spin coater onto a supporting substrate 1 comprising circular glass, and is dried to form a photothermal conversion layer 2. Further, a UV-hardening adhesive is applied using a spin coater onto the rear surface of a substrate for treatment 3 comprising a silicon wafer, to form an adhesive layer 4. Next, as shown in FIG. 17, the photothermal layer 2 of the supporting substrate 1 and the adhesive layer 4 of the substrate for treatment 3 are opposed and joined together, after which ultraviolet light 5 is used for irradiation from the side of the supporting substrate 1, hardening the adhesive layer 4 and bonding the supporting substrate 1 and the substrate for treatment 3.
And, as shown in FIG. 18, the substrate for treatment 3 immobilized on the supporting substrate 1 is ground to reduce the thickness. After grinding has ended, YAG laser light 6 is used for irradiation from the side of the supporting substrate 1, as shown in FIG. 19. By this means, the photothermal conversion layer 2b is decomposed, and the substrate for treatment 3a which has been reduced in thickness can be separated from the supporting substrate 1 without destroying the substrate (FIG. 20). Finally, as shown in FIG. 21, adhesive tape 7 for separation is affixed to the adhesive layer 4 remaining on the rear surface of the substrate for treatment 3a, and by peeling the adhesive tape 7, the adhesive layer 4 can be removed from the substrate for treatment 3a (FIG. 22).
In this technique, the supporting substrate 1 must have an outer shape larger than that of the substrate for treatment 3. This is because, if the outer shape of the supporting substrate 1 is equal to or smaller than the substrate for treatment 3, the adhesive protrudes, the adhesive layer 4 makes contact and adheres to the supporting substrate 1 without interposing of the photothermal conversion layer 2, and there are cases in which the supporting substrate 1 can no longer be separated from the substrate for treatment 3.    Patent Reference 1: Japanese Patent Application Laid-open No. 2004-64040
However, in this technique, when a supporting substrate 1 having a larger outer shape is joined together with a substrate for treatment 3 and the substrate for treatment 3 is ground, the photothermal conversion layer 2 protruding from the substrate for treatment 3 is also ground and removed, as shown in FIG. 18, and as a result, carbon black or other insoluble foreign matter 2d contained in the photothermal conversion layer 2 may re-adhere to the treatment surface of the substrate for treatment 1 and become a cause of defective external appearance (FIG. 19 through FIG. 22). Further, such defective external appearance is not limited to the above-described grinding treatment, and also occurs by, when performing treatment on the surface on the opposite side of the joined surface of the substrate for treatment, simultaneously treating the photothermal conversion layer protruding from the substrate for treatment, and thereby causing re-adhesion of foreign matter contained in the photothermal conversion layer to the substrate for treatment.